The adult smoking rate fell from 25 per cent in 1993 to 19 per cent in 2011. Helping that trend has been a boom in smoking bans in restaurants, bars and other public places, King noted.

The home is the main place that children are exposed to secondhand smoke. The CDC says secondhand smoke causes breathing problems in children, and causes an estimated 41,000 deaths among nonsmoking adults each year, mainly through heart disease and lung cancer.

In the new study, the government surveyed adults in about 200,000 U.S. homes in 2010 and 2011. The results were compared to previous versions of the same survey. Participants were asked if members of their household were allowed to smoke anywhere inside the home.

Over the past two decades, Utah consistently has had the largest proportion of homes where smoking is not allowed. About 94 per cent of Utah households fall in that category in the most recent survey. Kentucky and West Virginia were lowest, with both under 70 per cent.